U.S. patent application number 12/087221 was filed with the patent office on 2009-01-29 for high electric energy density polymer capacitors with fast discharge speed and high efficiency based on unique poly (vinylidene fluoride) copolymers and terpolymers as dielectric materials.
Invention is credited to Baojin Chu, Yingying Lu, Bret Neese, Qing Wang, Qiming Zhang, Xin Zhou.
Application Number | 20090030152 12/087221 |
Document ID | / |
Family ID | 38109142 |
Filed Date | 2009-01-29 |
United States Patent
Application |
20090030152 |
Kind Code |
A1 |
Zhang; Qiming ; et
al. |
January 29, 2009 |
High Electric Energy Density Polymer Capacitors With Fast Discharge
Speed and High Efficiency Based On Unique Poly (Vinylidene
Fluoride) Copolymers and Terpolymers as Dielectric Materials
Abstract
An improved charge or energy storage device having a dielectric
charge or energy storage layer including: (i) a copolymer or
terpolymer selected from P(VDF-CTFE), P(VDF-CFE), P(VDF-HFP),
P(VDF-CDFE), P(VDF-TrFE-CTFE), P(VDF-TrFE-CFE), P(VDF-TrFE-HFP),
P(VDF-TrFE-CDFE), P(VDF-TFE-CTFE), P(VDF-TFE-CFE), P(VDF-TFE-HFP),
and P(VDF-TFE-CDFE); or (ii) a polymer blend of PVDF homopolymer
with a copolymer selected from P(VDF-CTFE); P(VDF-CFE); P(VDF-HFP);
and P(VDF-CDFE); or (iii) a polymer blend of a PVDF homopolymer
with a terpolymer selected from P(VDF-TrFE-CTFE); P(VDF-TrFE-CFE);
P(VDF-TrFE-HFP); P(VDF-TrFE-CDFE); P(VDF-TFE-CTFE); P(VDF-TFE-CFE);
P(VDF-TFE-HFP); and P(VDF-TFE-CDFE); or (iv) a polymer blend of
copolymer selected from P(VDF-CTFE); P(VDF-CFE); P(VDF-HFP); and
P(VDF-CDFE); with a terpolymer selected from P(VDF-TrFE-CTFE);
P(VDF-TrFE-CFE); P(VDF-TrFE-HFP); P(VDF-TrFE-CDFE);
P(VDF-TFE-CTFE); P(VDF-TFE-CFE); P(VDF-TFE-HFP); and
P(VDF-TFE-CDFE).
Inventors: |
Zhang; Qiming; (State
College, PA) ; Chu; Baojin; (State College, PA)
; Zhou; Xin; (State College, PA) ; Lu;
Yingying; (State College, PA) ; Wang; Qing;
(State College, PA) ; Neese; Bret; (State College,
PA) |
Correspondence
Address: |
OHLANDT, GREELEY, RUGGIERO & PERLE, LLP
ONE LANDMARK SQUARE, 10TH FLOOR
STAMFORD
CT
06901
US
|
Family ID: |
38109142 |
Appl. No.: |
12/087221 |
Filed: |
December 18, 2006 |
PCT Filed: |
December 18, 2006 |
PCT NO: |
PCT/US2006/048258 |
371 Date: |
June 27, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60754497 |
Dec 28, 2005 |
|
|
|
Current U.S.
Class: |
525/199 |
Current CPC
Class: |
Y02T 10/7022 20130101;
H01G 4/186 20130101; Y02T 10/70 20130101 |
Class at
Publication: |
525/199 |
International
Class: |
C08L 27/16 20060101
C08L027/16 |
Claims
1-18. (canceled)
19. In a device for storing, and/or controlling, and/or
manipulating of charge and/or electric energy having a polymer film
as the dielectric layer, the improvement comprising: a dielectric
layer comprising: (i) a copolymer or terpolymer selected from the
group consisting of:
poly(vinylidene-fluoride/chlorotrifluoroethylene,
poly(vinylidene-fluoride/chlorofluoriethylene,
poly(vinylidene-fluoride/hexafluoropropylene,
poly(vinylidene-fluoride/chlorodifluoroethylene,
poly(vinylidene-fluoride/trifluoroethylene/chlorotrifluoroethylene,
poly(vinylidene-fluoride/trifluoroethylene/chlorofluoroethylene,
poly(vinylidene-fluoride/trifluoroethylene/chlorodifluoroethylene,
poly(vinylidene-fluoride/trifluoroethylene/chlorotrifluoroethylene,
poly(vinylidene-fluoride/trifluoroethylene/chlorofluoroethylene,
poly(vinylidene-fluoride/trifluoroethylene/hexafluoropropylene, and
poly(vinylidene-fluoride/trifluoroethylene/chlorodifluoroethylene;
or (ii) a polymer blend of poly(vinylidene fluoride) homopolymer
with a copolymer selected from the group consisting of:
poly(vinylidene-fluoride/chlorotrifluoroethylene;
poly(vinylidene-fluoride/chlorofluoriethylene;
poly(vinylidene-fluoride/hexafluoropropylene; and
poly(vinylidene-fluoride/chlorodifluoroethylene; or (iii) a polymer
blend of a poly(vinylidene fluoride) homopolymer with a terpolymer
selected from the group consisting of:
poly(vinylidene-fluoride/trifluoroethylene/chlorotrifluoroethylene;
poly(vinylidene-fluoride/trifluoroethylene/chlorofluoroethylene;
poly(vinylidene-fluoride/trifluoroethylene/chlorodifluoroethylene;
poly(vinylidene-fluoride/trifluoroethylene/chlorotrifluoroethylene;
poly(vinylidene-fluoride/trifluoroethylene/chlorofluoroethylene;
poly(vinylidene-fluoride/trifluoroethylene/hexafluoropropylene; and
poly(vinylidene-fluoride/trifluoroethylene/chlorodifluoroethylene;
or (iv) a polymer blend of copolymer selected from the group
consisting of: poly(vinylidene-fluoride/chlorotrifluoroethylene,
poly(vinylidene-fluoride/chlorofluoriethylene,
poly(vinylidene-fluoride/hexafluoropropylene, and
poly(vinylidene-fluoride/chlorodifluoroethylene; with a terpolymer
selected from the group consisting of:
poly(vinylidene-fluoride/trifluoroethylene/chlorotrifluoroethylene;
poly(vinylidene-fluoride/trifluoroethylene/chlorofluoroethylene;
poly(vinylidene-fluoride/trifluoroethylene/chlorodifluoroethylene;
poly(vinylidene-fluoride/trifluoroethylene/chlorotrifluoroethylene;
poly(vinylidene-fluoride/trifluoroethylene/chlorofluoroethylene;
poly(vinylidene-fluoride/trifluoroethylene/hexafluoropropylene; and
poly(vinylidene-fluoride/trifluoroethylene/chlorodifluoroethylene.
20. The device of claim 19, wherein mol % of
chlorotrifluoroethylene, or chlorofluoroethylene, or
hexafluoropropylene, or chlorodifluoroethylene in the copolymers
and terpolymers is in the range from 0 mol % to about 10 mol %.
21. The device of claim 19, wherein the mol % of trifluoroethylene
or tetrafluoroethylene in the terpolymers is in the range from 0
mol % to about 15 mol %.
22. The device of claim 19, wherein the composition of the
copolymer in the blend of poly(vinylidene fluoride homopolymer with
a copolymer or terpolymer in chlorotrifluoroethylene or
chlorofluoroethylene or hexafluoropropylene or
chlorodifluoroethylene is in the range of from 0 mol % to about 10
mol % and wherein the terpolymer in trifluoroethylene or
tetrafluoroethylene mol % is in the range from 20 mol % to 40 mol %
and wherein chlorotrifluoroethylene, or chlorofluoroethylene, or
hexafluoropropylene, or chlorodifluoroethylene is in the range of 3
to 10 mol %.
23. The device of claim 19, wherein the copolymer, terpolymer, and
the blends of poly(vinylidene fluoride) homopolymer or the
copolymer and the terpolymer films are uniaxially stretched to a
drawing ratio from 0.times. to 8.times. (zero times to 8 times) of
the original length.
24. The device of claim 19, wherein the copolymer, terpolymer, and
the blends of poly(vinylidene fluoride) homopolymer or the
copolymer and the terpolymer films are biaxially stretched to a
stretching ratio from 0.times. to 5.times. of the original
length.
25. The device of claim 19, wherein the stored electric energy
density of the copolymer and terpolymer films is at least about 10
J/cm.sup.3 under an electric field higher than 450 MV/m.
26. The device of claim 25, wherein the stored electric energy
density of the copolymer and terpolymer films is from about 10 to
about 30 J/cm.sup.3 under an electric field higher than 450
MV/m.
27. The device of claim 26, wherein the stored electric energy
density of the copolymer and terpolymer films is from about 10 to
about 20 J/cm.sup.3 under an electric field higher than 450
MV/m.
28. The device of claim 19, wherein the charge or energy storage
dielectric layer is polymer thin film capacitor.
29. The device of claim 28, wherein discharge time for release of
90% of the stored energy of the polymer thin film capacitor
(-0.1.about.IF) to a 1 kHz load is less than 1 ms.
30. The device of claim 28, wherein discharge efficiency is higher
than 85%.
31. The device of claim 19, wherein the polymer produces an energy
density at least about 10 J/cm.sup.3 to about 30 J/cm.sup.3.
32. The device of claim 19, wherein the polymer has a polarization
higher than 0.08 C/m.sup.2 at a breakdown field higher than 500
MV/m.
33. The device of claim 19, having a multilayer polymer dielectric
layer.
34. The device of claim 19, wherein the device is a capacitor.
35. The device of claim 19, wherein the device is a Field Effect
Transistor (FET).
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to a high electric
energy density polymer capacitor with fast discharge speed and high
efficiency. More particularly, the present invention relates to a
high energy density polymer capacitor based on a unique group of
PVDF based copolymers and terpolymers.
[0003] 2. Description of Related Art
[0004] The commercial and consumer requirements for compact and
more reliable electric power and electronic systems such as hybrid
electric vehicles and defibrillators have grown substantially over
the past decade. As a result, development of high electric energy
and power density capacitor technology has grown to become a major
enabling technology.
[0005] For a typical parallel plate capacitor, the capacitance C is
given by:
C=K.di-elect cons..sub.0A/t (1)
wherein K is the dielectric constant (relative permittivity), A is
the area and t is the thickness of the capacitor, and .di-elect
cons..sub.0 is a constant (vacuum permittivity, and
=8.85.times.10.sup.-12 F/m).
[0006] For linear dielectric materials, electric energy density
varies according to:
U.sub.e=1/2K.di-elect cons..sub.0E.sup.2 (2)
wherein E is the electric field in the capacitor.
[0007] Energy densities of nonlinear dielectrics must be derived
from the relationship
U.sub.e=.intg.EdD (3)
where D is the electric displacement.
[0008] Although ceramic-based dielectric materials usually display
very high dielectric constant (i.e., >1,000), the relatively low
breakdown field (<50 MV/m) and catastrophic failure in the
ceramic capacitors results in a low electric energy density (<1
J/cm.sup.3). On the other hand, although conventional polymers show
low dielectric constant (<5), the very high breakdown field
(>500 MV/m) generates a relatively high energy density. Thus,
for instance, biaxially oriented polypropylene (BOPP), even with a
dielectric constant of 2.2 (K=2.2), the high breakdown field
(.about.650 MV/m) produces a maximum electric energy density of
more than 4 J/cm.sup.3.
[0009] In PVDF based polymers, there are different molecular
conformations and a reversible change between the polar and
non-polar conformations can result in a large polarization change,
with the potential to reach a high energy density (see equation
(3)). However, the prior art does not teach how to control this
polarization change so that the maximum energy density (>20
J/cm.sup.3) allowed in this class of polymer can be achieved.
[0010] Accordingly, the present invention provides a new class of a
modified PVDF polymer, including PVDF based copolymers and
terpolymers in which the electric energy density at least 10
J/cm.sup.3 can be obtained and there is a possibility of reaching
energy density of 30 J/cm.sup.3. In addition, these polymer
capacitors can be charged and discharged with fast speed (in less
than 0.001 seconds) and with high efficiency (more than 85% of the
stored electric energy can be discharged to a load). These high
energy density polymer capacitors with fast discharge speed and
high efficiency will impact on a broad range of power electronics
and electric power systems such as these used in the
defibrillators, in the hybrid electric vehicles, and in the
electric weapons.
SUMMARY OF THE INVENTION
[0011] The present invention provides an improved charge or energy
storage device having an organic film as the charge or energy
storage layer. The improvement comprising:
[0012] a charge or energy storage layer comprising:
[0013] (i) a copolymer or terpolymer selected from the group
consisting of: P(VDF-CTFE), P(VDF-CFE), P(VDF-HFP), P(VDF-CDFE),
P(VDF-TrFE-CTFE), P(VDF-TrFE-CFE), P(VDF-TrFE-HFP),
P(VDF-TrFE-CDFE), P(VDF-TFE-CTFE), P(VDF-TFE-CFE), P(VDF-TFE-HFP),
and P(VDF-TFE-CDFE); or
[0014] (ii) a polymer blend of PVDF homopolymer with a copolymer
selected from the group consisting of: P(VDF-CTFE); P(VDF-CFE);
P(VDF-HFP); and P(VDF-CDFE); or
[0015] (iii) a polymer blend of a PVDF homopolymer with a
terpolymer selected from the group consisting of: P(VDF-TrFE-CTFE);
P(VDF-TrFE-CFE); P(VDF-TrFE-HFP); P(VDF-TrFE-CDFE);
P(VDF-TFE-CTFE); P(VDF-TFE-CFE); P(VDF-TFE-HFP); and
P(VDF-TFE-CDFE); or
[0016] (iv) a polymer blend of copolymer selected from the group
consisting of: P(VDF-CTFE); P(VDF-CFE); P(VDF-HFP); and
P(VDF-CDFE); with a terpolymer selected from the group consisting
of: P(VDF-TrFE-CTFE); P(VDF-TrFE-CFE); P(VDF-TrFE-HFP);
P(VDF-TrFE-CDFE); P(VDF-TFE-CTFE); P(VDF-TFE-CFE); P(VDF-TFE-HFP);
and P(VDF-TFE-CDFE).
[0017] The present inventors have discovered that a high electric
energy density with fast discharge speed (less than 0.001 seconds)
and high efficiency can be achieved in a unique group of polymer
capacitor materials, which combine the high breakdown field with
improved (matched) dielectric constant, phase stability of the
non-polar phase, and large polarization change between non-polar
and polar phases.
[0018] These copolymers and terpolymers capacitors can be used with
a broad range of power electronics including hybrid electric
vehicles and defibrillators for storing, controlling, and
manipulating electric charge, electric energy, and electric power
with high efficiency.
[0019] Further objects, features and advantages of the present
invention will be understood by reference to the drawings and
detailed description that follow.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a graph depicting discharge energy density of a
P(VDF-TrFE-CFE) 62/29/9 mol % terpolymer according to the present
invention as a function of the applied electric field.
[0021] FIG. 2 is a graph schematically illustrating the
relationship between the saturation electric field and electric
energy density. For the cases shown in FIG. 2, even though the
polymer in "1" has a higher dielectric constant than that in "2",
the lower saturation electric field in "1" leads to a lower energy
density.
[0022] FIGS. 3(a)-(c) are graphs having charging and discharging
curves measured using a Sawyer-Tower circuit at 10 Hz for (a)
unstretched PVDF, (b) uniaxially stretched PVDF, and (c) uniaxially
stretched P(VDF-CTFE) at 15 wt % CTFE.
[0023] FIG. 4 is a graph depicting discharged energy density of
unstrectched PVDF, uniaxially stretched PVDF homopolymer as a
function of electric field.
[0024] FIG. 5 is a graph depicting discharged energy density of
unstrectched P(VDF-CTFE), uniaxially stretched P(VDF-CTFE) at 15 wt
% CTFE as a function of electric field.
[0025] FIG. 6(a) is a graph depicting charging and discharging data
for a uniaxially stretched P(VDF-HFP) 90/10 wt %.
[0026] FIG. 6(b) is a graph depicting the corresponding discharged
energy density at different applied field levels.
[0027] FIG. 7 is a graph depicting discharging data of P(VDF-CTFE)
85/15 wet % under a field of 347 MV/m to a 100 kohm resistor load,
wherein the capacitance of the copolymer sample is at 0.5 nF.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] The present invention provides a device for storing, and/or
controlling, and/or manipulating of charge and/or electric energy
having a polymer film as the dielectric layer, such as, a
capacitor.
[0029] The polymer thin film can be copolymer or terpolymer
selected from P(VDF-CTFE), P(VDF-CFE), P(VDF-HFP), P(VDF-CDFE),
P(VDF-TrFE-CTFE), P(VDF-TrFE-CFE), P(VDF-TrFE-HFP),
P(VDF-TrFE-CDFE), P(VDF-TFE-CTFE), P(VDF-TFE-CFE), P(VDF-TFE-HFP),
P(VDF-TFE-CDFE) is provided, wherein:
[0030] CTFE: chlorotrifluoroethylene;
[0031] CFE: chlorofluoroethylene;
[0032] HFP: hexafluoropropylene;
[0033] CDFE: chlorodifluoroethylene;
[0034] TrFE: trifluoroethylene; and
[0035] TFE: tetrafluoroethylene.
[0036] The mol % of CTFE, or CFE, or HFP, or CDFE in the copolymers
and terpolymers is in the range between 0 mol % and 10 mol %.
[0037] The mol % of TrFE or TFE in the terpolymers is in the range
between 0 mol % and 15 mol %.
[0038] Also provided is a polymer blend of PVDF homopolymer with a
copolymer selected from the group consisting of: P(VDF-CTFE);
P(VDF-CFE); P(VDF-HFP); and P(VDF-CDFE) or a polymer blend of a
PVDF homopolymer with a terpolymer selected from the group
consisting of: P(VDF-TrFE-CTFE); P(VDF-TrFE-CFE); P(VDF-TrFE-HFP);
P(VDF-TrFE-CDFE); P(VDF-TFE-CTFE); P(VDF-TFE-CFE); P(VDF-TFE-HFP);
and P(VDF-TFE-CDFE), including, for example, polymer blends with
either a PVDF homopolymer or a copolymer and a terpolymer. The
copolymer is selected from one of the following:
[0039] P(VDF-CTFE);
[0040] P(VDF-CFE);
[0041] P(VDF-HFP); and
[0042] P(VDF-CDFE).
[0043] Additionally provided is a polymer blend of copolymer
selected from the group consisting of: P(VDF-CTFE); P(VDF-CFE);
P(VDF-HFP); and P(VDF-CDFE); with a terpolymer selected from the
group consisting of: P(VDF-TrFE-CTFE); P(VDF-TrFE-CFE);
P(VDF-TrFE-HFP); P(VDF-TrFE-CDFE); P(VDF-TFE-CTFE); P(VDF-TFE-CFE);
P(VDF-TFE-HFP); and P(VDF-TFE-CDFE). The terpolymer is selected
from the following:
[0044] P(VDF-TrFE-CTFE);
[0045] P(VDF-TrFE-CFE);
[0046] P(VDF-TrFE-HFP);
[0047] P(VDF-TrFE-CDFE);
[0048] P(VDF-TFE-CTFE);
[0049] P(VDF-TFE-CFE);
[0050] P(VDF-TFE-HFP); and
[0051] P(VDF-TFE-CDFE).
[0052] The composition for the copolymer in a blend of PVDF
homopolymer or a copolymer and a terpolymer is at CTFE or CFE or
HFP or CDFE in the range of 0 mol % to 10 mol % and the terpolymer
is at TrFE or TFE mol % in the range from 20 mol % to 40 mol % and
at CTFE, or CFE, or HFP, or CDFE in the range of 3 to 10 mol %.
[0053] The copolymer, terpolymer, and blends of PVDF homopolymer or
a copolymer and a terpolymer films are uniaxially stretched to a
drawing ratio from 0.times. to 8.times. (zero times to 8 times) of
the original length.
[0054] The copolymer, terpolymer, and blends of PVDF homopolymer or
a copolymer and a terpolymer films are biaxially stretched to a
stretching ratio from 0.times. to 5.times. of the original
length.
[0055] The stored electric energy density of these copolymer and
terpolymer films at least about 10 J/cm.sup.3 under an electric
field higher than 450 MV/m, preferably between about 12 to about 30
J/cm.sup.3 under an electric field higher than 450 MV/m, and more
preferably between about 12 to about 22 J/cm.sup.3 under an
electric field higher than 450 MV/m.
[0056] The discharge time (release of 90% of the stored energy) of
a polymer thin film capacitor (.about.0.11 .mu.F) to a 1 kohm load
should be less than 1 ms.
[0057] The discharge efficiency, as defined by the ratio of the
discharged energy density to the stored energy density (which can
be directly derived from FIG. 3 and FIG. 6 using equation 3),
should be better than 85% for 1 ms discharge time.
[0058] The polymer possesses a stable non-polar phase after the
uniaxial drawing of the film to more than 5.times. or application
of electric field of higher than 400 MV/m at temperatures above
50.degree. C.
[0059] The present inventors have discovered based on molecular
structure consideration and dielectric constant/electric
polarization/saturation electric field relationship that a high
electric energy density with high discharge efficiency and fast
discharge time can be achieved with a unique class of polymer
capacitor materials, which combine the high breakdown field with
improved (marched) dielectric constant and phase stability of the
non-polar phase.
[0060] For example, in recently developed relaxor ferroelectric
polymers, i.e., poly(vinylidene-fluoride/trifluoroethylene)
(P(VDF-TrFE)) based terpolymers and high energy electron irradiated
P(VDF-TrFE) copolymers, a room temperature dielectric constant of
higher than 50 has been achieved. Consequently, under a field of
350 MV/m, an electric energy density of 10 J/cm.sup.3 can be
obtained (see FIG. 1).
[0061] On the other hand, due to the nonlinear dielectric behavior,
the relaxor ferroelectric polymers show polarization saturation (as
indicated by a reduction of the effective dielectric constant with
applied field amplitude), which limits the further increase of the
electric energy density to far beyond 10 J/cm.sup.3 level.
[0062] In PVDF based polymers, there are different molecular
conformations and a reversible change between the polar and
non-polar conformations can result in a large polarization change,
with the potential to reach a high energy density as suggested by
equation (3). However, the prior art does not teach how to control
this polarization change so that the maximum energy density allowed
in this class of polymer can be achieved.
[0063] Accordingly, the present invention provides a new class of a
modified PVDF polymer, including PVDF based copolymers and
terpolymers in which the energy density at least 10 J/cm.sup.3 can
be obtained and there is a possibility of reaching energy density
of 30 J/cm.sup.3.
[0064] These high energy density polymer capacitor materials will
impact on a broad range of power electronics and electric power
systems such as these used in the hybrid electric vehicles and in
the defibrillators.
[0065] In all the dielectric materials, there exists a polarization
saturation, that is, the polarization level can't be increased
further even applying higher electric fields (see FIG. 2). For PVDF
based polymers, the highest polarization can be reached is about
0.1 C/m.sup.2 and the breakdown field can be more than 600 MV/m. If
the polarization saturation occurs at 500 MV/m, the energy density,
following FIG. 2, will reach 25 J/cm.sup.3. If the polarization
saturation occurs at 600 MV/m, the energy density, following FIG.
2, will further reach 30 J/cm.sup.3.
[0066] For such a dielectric materials, the dielectric constant is
20, lower than the relaxor ferroelectric polymer (K>50). This
analysis indicates that to achieve higher energy density in PVDF
based polymers, a dielectric constant near 20 would be
preferred.
[0067] In PVDF homopolymers, the room temperature dielectric
constant can reach 12 and a breakdown field of higher than 500 MV/m
has been shown, indicating a potential to achieve an electric
energy density .about.15 J/cm.sup.3. If PVDF polymer can maintain
its nonpolar phase (the .alpha.-phase) after applying a high
electric field, the material would be attractive for the high
energy density capacitors. However, many earlier studies have shown
that the .alpha.-phase of PVDF polymer will be gradually converted
to the .beta.-phase under high electric field (.about.500
MV/m).
[0068] Stretching PVDF films can also result in a .alpha.-to-.beta.
phase conversion. In the .beta.-phase, the energy density of the
polymer is much lower due to the remanent polarization. In
addition, the dielectric loss will increase due to the presence of
the ferroelectric phase. In this sense, PVDF homopolymer is not an
ideal dielectric material for the high energy capacitors.
[0069] Shown in FIGS. 3(a), 3(b) and 4 is the discharged energy
density of PVDF versus the applied field E. Although an energy
density of higher than 10 J/cm.sup.3 can be reached, there are
indications that the polarization switching process is also
accompanied by the .alpha.-to-.beta. phase conversion (relative
large polarization hysteresis), which is not desirable and suitable
for long term practical and reliable use in various electric and
electronic systems.
[0070] On the other hand, by introducing small amount of another
monomer into the PVDF polymer to expand the inter-chain space and
break-up the dipole coherence in the polymer, the .alpha.-phase
will be favored and stabilized even under mechanical stretching.
After the application of very high electric field (>500 MV/m),
the polymer can still return to the nonpolar phase, which is
distinctively different from the PVDF homopolymer.
[0071] These considerations indicate that a few PVDF copolymers
with bulky co-monomers such as chlorotrifluoroethylene (CTFE),
chlorofluoroethylene (CFE), and hexafluoropropylene (HFP), and
other similar monomers, have the potential to achieve high electric
energy density.
[0072] Furthermore, by stretching the copolymers such as
P(VDF-CTFE), P(VDF-HFP), and P(VDF-CFE), the polymer chain
directions are aligned to perpendicular to the applied field so
that the polarization level can be increased and consequently the
higher energy density may be obtained.
[0073] As shown in FIG. 3(c) and FIG. 5, for an uniaxially
stretched P(VDF-CTFE) 85/15 wt % copolymer, the breakdown field can
reach more than 570 MV/m and an energy density of 17 J/cm.sup.3 can
be obtained. The dielectric constant of this stretched copolymer at
low electric field is about 15.
[0074] Shown in FIG. 6 is the energy density data for a uniaxially
stretched P(VDF-HFP) 90/10 wt % copolymer. The breakdown field is
525 M/m and an energy density of near 12 J/cm.sup.3 is achieved. It
is also observed that by either uniaxially or biaxially stretching
these films, the electric breakdown field can be increased.
[0075] The data in FIGS. 5 and 6 also reveal that even at the
highest field level, the polarization of these polymers is not
saturated. In other words, the saturation polarization of these
polymers is higher than 0.09 C/m.sup.2. For example, in FIG. 3,
extrapolating the polarization to 0.1 C/m.sup.2 and the field to
650 MV/m, an electric energy density of 24 J/cm.sup.3 can be
achieved. On the other hand, one can also increase the dielectric
constant so that the saturation of 0.1 C/m.sup.2 is reached at 500
MV/m or 550 MV/m, a lower field than 650 MV/m. Consequently,
smaller energy density will be obtained (.about.20 J/cm.sup.3).
[0076] For the P(VDF-CTFE) copolymer and other similar ones (where
the 2.sup.nd monomer is bulkier in size than VDF to expand the
inter-chain space, and favoring the TGTG' conformation), one can
introduce small amount of TrFE to raise the dielectric constant of
these polymers, the mol % of TrFE can be in the amount 10 mol % or
less. In addition, polymer blends of P(VDF-CTFE) or similar
copolymer with the relaxor ferroelectric terpolymer of
P(VDF-TrFE-CFE) (CFE: chlorofluoroethylene) and similar terpolymers
can also lead to higher energy density.
[0077] FIG. 7 shows the discharge data of the P(VDF-CTFE) 85/15 wt
% copolymer into a 100 kohm load. As can be seen, the stored
electric energy can be released within very short time (less than
0.1 ms) which demonstrates that this class of high energy density
capacitor can be operated to frequencies higher than 10 kHz.
[0078] A variety of PVDF based copolymers are commercially
available. For example, P(VDF-CTFE) and P(VDF-HFP) can be purchased
from Solvay, Arkema, and 3M. Other copolymers that are not
commercially available can be synthesized using the suspension
polymerization methods.
[0079] Thus, P(VDF-CFE), P(VDF-CDFE), P(VDF-TrFE-CTFE),
P(VDF-TrFE-CFE), P(VDF-TrFE-HFP), P(VDF-TrFE-CDFE),
P(VDF-TFE-CTFE), P(VDF-TFE-CFE), P(VDF-TFE-HFP), P(VDF-TFE-CDFE)
can be synthesized using a suspension polymerization process using
an oxygen-activated initiator.
[0080] The polymer blends can be fabricated by one of several
methods, either by solution blending methods, melt method,
extrusion method, or by any other convenient method which can blend
the two polymers into a blend.
[0081] Thus, in the first step, each polymer used in the blend is
synthesized or purchased from a commercial source.
[0082] In the solution method, the polymers with proper weight
ratios are dissolved in a solvent such as methyl ethyl ketone or
any suitable solvent that can dissolve the two polymers. The
solution is then poured onto a glass plate and, after the
evaporation of the solvent, a polymer film is formed.
Alternatively, tape casting method can be used.
[0083] In the melt method, the two polymers in a proper wt % ratio
are heated at near or above the melting temperatures of both
polymers to obtain a uniform melt and thereafter, the melt is
pressed under a stress to form a polymer film.
[0084] In the extrusion method, the two polymers in a proper wt %
ratio are fed to the extruder to be processed to form a polymer
film.
[0085] The present invention has been described with particular
reference to the preferred embodiments. It should be understood
that the foregoing descriptions and examples are only illustrative
of the invention. Various alternatives and modifications thereof
can be devised by those skilled in the art without departing from
the spirit and scope of the present invention. Accordingly, the
present invention is intended to embrace all such alternatives,
modifications, and variations that fall within the scope of the
appended claims.
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